The present invention relates to multiple servo actuation systems, and more particularly concerns the balancing or equalizing of forces exerted by two or more actuators on a common member.
For various reasons, such as, for example, the length of a driven member, or to obtain redundant operation, servo actuation systems may employ two or more actuators connected at mutually spaced points of a member that is to be driven by the two actuators acting in unison. Such multiple actuator arrangements are often found in systems for operation of aircraft control surfaces, such as ailerons, flaps, elevators and the like. Frequently, where a pair of actuators is employed to drive a common member, the two are controlled by a common command input and are arranged to nominally provide identical forces to actuate the common driven member. In order to cause the dual servo controlled actuators to exert the same force at two different points on the driven member, the two systems are made as nearly identical as possible in all respects. Thus, electro hydraulic servo valves that control hydraulic actuators are made to be identical, as are the actuators and their electronic circuits. Particular attention is also directed to physical positioning and mounting of the actuators in an attempt to obtain equal strokes of the two in response to a common command and, therefore, equal displacements of the two areas of the common driven member that are connected to the actuators. Thus, in "rigging" of the servo actuators, physical mounting, location of mounting bolts and the like, must be carefully controlled, with parts shimmed or physically adjusted as necessary. Nevertheless, in spite of all effort made to equalize the dual actuators, various errors, including errors in valve alignment, electronic offsets, physical mounting structures, thermal differences, and different aging of parts, contribute to cause a net difference in the operation of the two servo controlled actuators. Because of this difference there occurs a condition known as "force fight", wherein the common command signal, fed to both of the electro hydraulic servo valves that control the two actuators, actually causes a different magnitude of stroke in the two actuators, which therefore apply different forces to the driven member. With different forces applied at two different points of the common member, forces exerted by the actuators tend to oppose each other to some extent, resulting in a highly undesirable operation, and, at the very least, in a decrease in the effectivity of actuation.
Various arrangements have been attempted to minimize this force fight in dual actuation systems. Some have attempted to continuously generate error signals during system operation and feed back such error signals during system operation to decrease the imbalance of the servo actuators. However, such a feedback signal during operation of the system may possibly cause instabilities and effect oscillation, and in some cases may cause more damage than improvement.
Accordingly, it is an object of the present invention to provide for correction or decrease of force fight in a multiple servo actuation system while minimizing or avoiding above-mentioned problems.